A second-generation mobile communication system GSM (Global System for Mobile Communications) includes a packet-switched data transmission enhancement called GPRS (General Packet Radio Service). In the GPRS system, the user data transmission between the mobile station MS and the base station system of the GSM network over the radio interface Um is performed in accordance with the conventional GSM protocol, the data being transferred over the radio interface Um as radio link layer packet data units (RLC PDU). The protocol stacks of the mobile station MS, on one hand, and of the serving GPRS support node SGSN located in the network, on the other hand, include the higher protocol layers defining further the data packet transfer. These protocol layers include two layers, SNDCP (Sub-Network Dependent Convergence Protocol) and LLC (Logical Link Control).
User data transmitted in the SNDCP layer is segmented to one or more SNDC data units, whereby the user data and the header field associated with it can optionally be compressed. The SNDC data units are transmitted in LLC frames, which are associated with address and checking information essential to the data transmission, and in which frames the SNDC data units can be encrypted. The function of the LLC layer is to maintain the data transmission connection between the mobile station MS and the serving node SGSN and to retransmit the damaged frames. For the transmission over the radio interface Um these LLC frames are divided into radio link layer packet data units RLC PDU. On the reception, steps to the opposite direction are performed, i.e. RLC PDUs are shifted to the LLC layer and further to the SNDCP layer, while simultaneously checking the structure and the integrity of the data according to each layer.
A problem may occur, when transferring the RLC PDUs to the LLC layer. In the GPRS packet transfer, the received/sent LLC PDU sizes (without LLC headers) according to the GPRS standards can vary from 1 to 1520 bytes. RLC layer delivers every packet separately to LLC layer. For example, if the transferred file size is 1000 bytes, this can generate only one packet (LLC PDU) or, in the worst case, even 1000 packets (1 byte in each LLC PDU). Especially from the mobile station's MS viewpoint, this sets high requirements for MCU (Master Control Unit) processor to handle a largely varying load.
When the RLC layer of the mobile station receives many small RLC packets from the network, the RLC layer delivers packets to LLC layer as LLC packets comprising typically several RLC packets as concatenated into one LLC packet. Normally the LLC layer and the SNDCP layer would receive the packets and transfer them upwards. If the RLC layer has higher priority than the other layers, the RLC layer uses most of the processing power, resulting in that the LLC layer and the SNDCP layer can subsequently handle only a small subset of packets. The RLC layer is working in full speed, inserting more and more packets in the stack between the RLC and the LLC. Finally the mobile station runs out of memory and crashes. Moreover, there exists no procedure to tell the network to slow down transmission speed in the RLC layer.